1. Field of the Invention
The present invention relates to a steering control system for a motor vehicle such as, for example, a motorcar.
2. Discussion of the Related Art
In the field of steering devices for vehicles and especially, in the field of steering devices for motorcars, for higher performance, there has recently been developed a system incorporating a so-called “variable steering angle ratio converting mechanism” which is capable of varying the conversion ratio of a handle steering angle to a wheel steering angle (steering conversion ratio) in dependence on the traveling state of the vehicle without keeping the manipulation angle of a steering handle (handle manipulation angle) and a wheel steering angle fixed at a one-to-one ratio. For example, the vehicle speed can be exemplified as representing the vehicle traveling state, and in the system, the stability in a high speed traveling can be ensured by making the steering conversion ratio small so that the steering angle does not increase sharply with increase in the handle manipulation angle during the high speed traveling. During a slow speed traveling, on the contrary, the steering conversion ratio is made larger so that the number of rotations needed to make a full turn of the handle is decreased. This advantageously makes it possible that the manipulation with a large steering angle as required for, e.g., garaging, in-line parking, pulling-over to the kerb or the like can be performed very easily.
For example, as disclosed in Japanese unexamined, published patent application No. 11-334604 (1999-334604), a mechanism for changing the steering conversion ratio is constituted by directly connecting a handle shaft and a wheel steering shaft with a gear type transmission unit whose gear ratio is variable. However, this type of the mechanism involves a drawback that the gear ratio changing mechanism of the gear type transmission unit is complicated in construction. Another type of the mechanism has been proposed in, for example, Japanese unexamined, published patent application No. 11-334628 (1999-334628), wherein a wheel steering shaft is rotationally driven by a motor. More specifically, a wheel steering angle finally needed is calculated through a computer processing based on a steering conversion ratio which is determined in dependence on a handle manipulation angle detected by an angle detection section and the traveling state of the vehicle, and the wheel steering shaft is rotationally driven by the motor with the handle shaft being disconnected mechanically therefrom thereby to bring the wheel steering angle into coincidence with that calculated.
In the foregoing steering control method, the frequency in operation of the motor for rotationally driving the wheel steering shaft increases very high due to the frequent handle manipulations performed during the driving of the vehicle. Accordingly, in improving the maintenance capability of the motorcar practicing the control method of this kind, it is important to ensure that the motor can operate stably over a long period of time. For example, motors tend to deteriorate their performances when continued to be operated for a long period of time in such a state that an excess current or overcurrent is applied thereto due to an overload or the like. Therefore, it becomes effective to use the motor under a moderate protection by monitoring the current value applied to the motor by means of a current sensor and by restraining the electric current to the motor when the state of overcurrent is detected.
However, where some reason causes the current sensor become unable to operate normally, it becomes impossible to restrain the application of electric current to the motor as expected, whereby the motor tends to run for a shorter life.